How do we get this equation ? ?

h=v^2÷r

is the equation that represents the Joule's law of heating.

Joule's law of heating defines the heat generated by any current flowing conductor is directly proportional to  

1. Square of Current (I²),

2. Resistance of the conductor (R)

3. Time for which current is passed (t)

Hence, Heat generated = .....................(1)

By Ohm's Law, the potential difference (V) across a conductor is directly proportional to the current(I) flowing through it. The constant of proportionality is termed as resistance of the conductor (R).

...............................................(2)

From (2), Current (I) can be rewritten as

........................................................(3)

Substituting (3) in (1), we get